In optical communication systems, such as those operating according to spread-spectrum techniques, a transmitted signal is spread over a frequency band that is much wider than the bandwidth of the information being transmitted. Two techniques commonly used in spread-spectrum systems are frequency hopping and direct sequence (DS) modulation. Frequency hopping involves shifting the carrier frequency in discrete increments, in a pattern dictated by a pseudo-random code. In direct sequence modulation, each bit of an information-bearing signal is modulated by a higher frequency, pseudo-random code signal. The modulation may simply comprise reproducing the input code signal when the information bit is one, and inverting the code signal when the information bit is zero. Each bit of the code signal, or each bit of the product signal obtained by modulating the information-bearing signal with the code signal is referred to as a “chip.”
In a system using direct sequence modulation, the chip rate, i.e., the frequency of the pseudo-random code signal, is typically much higher than the bit rate of the information-bearing signal. The bandwidth occupied by the transmitted signal is directly determined by the chip rate. A receiver in a direct sequence modulated communication system includes means for producing the same pseudo-random code signal as that used by the transmitter, in the same time epoch. The code signal is employed to decode the transmitted data and extract the information-bearing signal, even in the presence of noise or jamming.
Applications of spread-spectrum systems are various and generally depend upon characteristics of the codes being employed for band spreading and other factors. In direct sequence systems, for example, where the code is a pseudo-random sequence, the transmitted signal acquires the characteristics of noise, making the transmission indiscernible to any eavesdropper who is incapable of decoding the transmission. In this regard, system sensitivity to interference is fundamentally altered. The use of noise-like modulation carrier signal, occupying the same frequency spectrum as all other users, creates effective noise that equals the sum of all the other user signals. Thus, the effective signal-to-noise (S/N) ratio at the receiver is increased because the noise is no longer a worst-case interference signal (as previously required), but instead the average interference of the summed signals from the other users.
In addition to the benefits of making the transmission indiscernible to eavesdroppers, and decreasing the sensitivity to system receivers, spread-spectrum techniques can also increase the data channel density available in a frequency channel. By spreading each bit of an information-bearing signal over a bandwidth of frequencies determined by the pseudo-random code signal, the amount of data that can be transmitted over a given frequency channel is increased over traditional narrow-band systems.
Whereas spread-spectrum communications provide a large number of benefits over traditional communication techniques, conventional spread-spectrum communications systems also have their limitations. In this regard, just as the effective noise in a channel is the sum of signals on the channel, the energy density of each channel has an upper maximum where the waveguide for the channel becomes saturated. Additionally, in optical transmission, a phenomenon known as polarization dispersion occurs when optical signals travel over long distances. Polarization dispersion is an effect caused in light that travels in multiple polarization modes. When a waveguide, such as an optical fiber, is asymmetric in all directions, the light traveling along one polarization can end up traveling at a speed different than light traveling in another direction. If the light spreads enough, the signal can overlap with other light signals and, thus, corrupt the both signals.